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Ash dieback disease research. Researcher with a portable DNA analyser (Genie II) sampling an ash tree for signs of ash dieback disease. This disease is caused by the fungus Chalara fraxinea, also known as Hymenoscyphus pseudoalbidus in its fruiting stage. It causes necrosis (tissue death) in the tree's stems, causing dieback of the leaves and shoots. The disease can be lethal, especially in young trees. This fungus was first found in the UK in 2012, and it is feared that the spread of the disease may affect millions of ash trees in the country's woodlands. Photographed in the UK by the Food and Environment Research Agency.